Process for the production of permanent magnets

ABSTRACT

Permanent magnets having high coercivity forces and remanence are produced by melting and cooling a charge of magnetic material in a strong magnetic field.

United States Patent [191 Kuhlmann-Schiifer PROCESS FOR THE PRODUCTION OF PERMANENT MAGNETS [75] Inventor: Wilhelm Kuhlmann-Schiifer,

Hannover, Germany [73] Assignee: Preussag Aktiengesellschaft,

Hannover, Germany [22] Filed: June 15, 1972 [21] Appl. No.: 264,399

52 U.S. 01.; ..164/49,164/50,164/122 s1 rm. CI B22d 27/02 [58] Field of Search 164/49, 51, 250, 251

[56] References Cited UNITED STATES PATENTS 3,464,812 9/1969 Utech et al. 164/49 X 2,380,616 Snoeket a1. 164/57 2,398,018 4/1946 Linley et a1 164/49 2,773,923 12/1956 Smith 164/251 X 3,163,523 12/1964 Porter et a1 164/51 X 3,246,373 4/1966 Lyman 164/49 3,322,183 5/1967 Johnston et al 164/49 FOREIGN PATENTS OR APPLICATIONS 90,563 2/1897 1 Germany 164/50 979,695 12/1950 France 164/49 Primary Examiner-R. Spencer Annear Attorney, Agent, or Firm-Molinare Allegretti, Newitt & Witcoff [57] ABSTRACT Permanent magnets having high coercivity forces and remanence are produced by melting and cooling a charge of magnetic material in a strong magnetic field.

9Claims, 1 Drawing Figure 1' PROCESS FOR THE PRODUCTION OF PERMANENT MAGNETS.

BACKGROUND OF THE INVENTION The invention relates to a process for the production of permanent magnets. t

It is known to sinter C0,,R magnets from fine particles of magnetic material in the production of these materials. This production is carried out in various stages, namely, the mixing of the raw materials in a fluid, and filtering, drying, pre-sintering, grinding, pressing, final sintering, and forming the resultant product. This sintering process is relatively complicated.

When carrying out the sintering process, it is known to produce a magnetic field in the charge during the pressing part of the operation in order to obtain more favorable magnetic properties. The purpose of said magnetic field is to substantially align the axes of easiest magnetizability of individual powder particles with v the magnetic field. In order to permit this, the powder must be introduced into the mold either-in a very dry state or in a suspension in water, which further increases the production costs involved. It is also known that optimum values can be achieved when using a sintering process in a magnetic field if each powder particle is a single crystal of suitable size. Therefore, extremely fine grinding is a prerequisite if optimum values are tobe achieved. (See All. Stuijts et 211., Phil. Tech. Rev. 16, page 141, 1954, the teachings of which are incorporated herein by reference.)

To improve the magnetic properties, and particularly to produce a preferred direction of magnetization, it is known in the case of ferrites containing Co additives to cool the charge after final sintering in a magnetic field between the Curie temperature and about 150C. (See M. Kornetzki et al., Siemens-Zeitung 29, page 434, 1955, the teachings of which are incorporated herein by reference.)

It is also known to grow monocrystals in order to improve magnetic properties. But, in fact, the Weiss domains usually have no specific association with the geometry of the monocrystal, so that an effective improvement in magnetic properties cannot be achieved thereby.

SUMMARY OF THE INVENTION The object of the invention is to provide a simple process for the production of permanent magnetsv having improved magnetic properties, particularly, magnets having high coercivity'forces or high remanences.

The object of this invention is achieved by a process for the production of permanent magnets wherein a charge of magnet material is melted and cooled in a strong magnetic field. It is particularly convenient and preferred toeffect the melting of the charge in a migrating zone, the transition from'the liquid phase to the rigid phase occurring in the center of the strong magnetic field.

With the process of the present invention, the aligning of the Weiss zones is effected by a magnetic field when the individual molecules of the magnet material exhibit their maximum mobility, i.e., whenthe material is in the melted, fluid state. After cooling and consolidation of the material, the material isgiven a preferred direction of magnetization in the directionof the lines of flux ofthe applied magnetic field, so that the magnetic properties are substantially improved. At worst,

the Weiss domains are at a small angle relative to one another and are usually uniformly aligned. As a result, the magnetic properties are at an optimum.

A further development in the process of this invention is that there is conducted through the charge a current of such intensity that it is capable of heating the charge to a temperature above the melting point. The chargeis then cooled, after passing through the melting zone, to a temperature below the'melting point. Thus, because of the effectiveness of the cooling of the charge, the envelope about the charge can be kept particularly thin so that the inner diameter of an electromagnet, preferably a superconductive magnet, can be correspondingly small and the field intensity in the charge particularly great. Preferably, the charge is surrounded in the region before the melting zone by a heat-insulating envelope.

Prior to this invention, high coercivity forces of the order of magnitude of 200 kilogauss have beenonly sporadically found with small particles of Co,-,R (R is a radical). With particles of larger dimension, it has been impossible to produce permanent magnets from Co R. The process of the present invention-facilitates the transformation 'of large Co R particles into permanent magnets, thus providing an economical means of producing large magnets with coercivity forces of the order of magnitude of 200 kilogaussand with high-remanences. These properties afford considerable advantages in electric motors, frictionless suspensions, magnetic pads, magnetic separators, elementary particle accelerators, quadripole systems and the like.-

Since, depending on the type of magnet material used, a reaction may take place with the gas of the surrounding atmosphere, it is expedient toconduct the melting and particularly the zone melting operation under an inert gas atmosphere. Naturally, the process also permits the production of magnetic'monocrystals.

The invention will be explained indetail with reference to the attacheddrawi'ng.

The drawing shows, in principle, a device for carrying out zone melting. Between two electrodes 1 and 2', there is a charge 3 which is, for example, in the form of a Co R rod which is to be subjectedto zone melting. The electrodes 1' and 2 can move with the charge 3 in the direction of arrows 5 through an envelope 4 cooled with water. In the drawing, the direction of movement is vertical and the other parts are arranged correspondingly. It is possible to have this direction with cruciblefree zone melting, wherein the charge is brought to a liquid state only over a short region, and is held together by the surface tension of the liquid. However, the charge may also be held in a concentric ceramic tube 6made of, for example, Y O :in the case of (305R, so that it can be brought to the liquid state over a longer region. In the device shownin the drawing,'th e ceramic tube 6 is surrounded by a secondceramic tube 7. The intervening space between the two ceramic tubes 6 and 7 is filledwith heat-insulating ceramic powder8. Between the ceramictube7 and the envelope 4, there is ceramic wool 9for further heat insulation. It will be noted that the envelope 4*is widened internally in theregion' of the'ceramic tubes 6 and 7 to provide a space for the ceramic tubes.

The envelope 4 is surrounded by a superconductive magnet 10, the center of which is in the region of the transition from the liquid phase to the solid phase, said region lying in the direction of movement of the charge 3 as it moves in the direction of the arrows 5, and between the ends of the ceramic tubes 6 and 7 and the casing 4. Subsequently, the casing 4 is again narrowed. This means that the center of the superconductive magnet is situated in the transition region between heat insulation and heat dissipation.

The superconductive magnet 10 consists of a plurality of superconductors l1 (so-called pan cakes), consisting, for example, of Nb Sn or vGa which are cooled by a radiation shield 12 and a helium bath 13.

To carry out the process of the present invention, an electric current of sufficient intensity is sent through the charge so that the charge 3 is liquefied within the the art. In the aforesaid heat-insulated zone, the strong magnetic field of the superconductive magnet 10 effects substantial alignment of the Weiss domains, or gives them a parallel form. Since this alignment is particularly important when the re-solidification is effected in the region between the downstream end of the ceramic tubes 6 and 7 and the beginning of the narrowed portion of the envelope used for cooling, this region is located in the center of the superconductive magnet 10. After solidification, the permanent magnet thus produced-is given a preferred direction of magnetization in the direction of the lines of flux of the superconductive magnet 10. Therefore, the magnetic properties obtained are good, with especially high coercivity force and remanence.

As used herein, R in Co R refers to a rare earth. See for example, 1971 lntermag Conference; IEE Transactions on Magnetics, The Preparation of RG Permanent Magnet Alloys, page 423 (1971) the teachings of which are incorporated by reference herein. Further, the strong magnetic field used in producing the permanent magnets is held at a value of about 50-250 kilogauss and is maintained at that value until solidification occurs.

I claim:

l. A process for production of a permanent magnet from a charge of magnetic material comprising the steps of:

heating said charge to provide a melting zone along a portion of the length of said charge, said melting zone having a liquid-to-solid interface; causing said melting zone to pass through said charge; and

continuously subjecting said interface of said melting ing step further includes insulating a second portion of c said charge with a heat-insulating envelope, said melting zone substantially aligning with said heat-insulating envelope.

5. A process as claimed in claim 4 wherein said causing step includes moving said charge relative to said envelope. 1

6. A process as caimed in claim 1 further comprising the steps of placing said charge in an air-tight container and filling said container with an inert gas.

7. A process as claimed in claim 1 wherein said magnetic material of said charge is Co -,R, wherein R is a rare earth metal.

8. A process as claimed in claim 1 wherein said charge is contained in a crucible of Y O and said magnetic material is Co R.

9. A process as claimed in claim 1 where the strength of said magnetic field is at least 50 kilograms. 

1. A process for production of a permanent magnet from a charge of magnetic material comprising the steps of: heating said charge to provide a melting zone along a portion of the length of said charge, said melting zone having a liquidto-solid interface; causing said melting zone to pass through said charge; and continuously subjecting said interface of said melting zone to a magnetic field.
 2. A process as claimed in claim 1 wherein said liquid-to-solid interface is positioned in the center of said magnetic field.
 3. A process as claimed in claim 1 wherein said heating step includes passing a current through said charge to heat said charge to a temperature above the melting point of said magnetic material and cooling a portion of said charge to a temperature below the melting point.
 4. A process as claimed in claim 3 wherein said heating step further includes insulating a second portion of said charge with a heat-insulating envelope, said melting zone substantially aligning with said heat-insulating envelope.
 5. A process as claimed in claim 4 wherein said causing step includes moving said charge relative to said envelope.
 6. A process as caimed in claim 1 further comprising the steps of placing said charge in an air-tight container and filling said container with an inert gas.
 7. A process as claimed in claim 1 wherein said magnetic material of said charge is Co5R, wherein R is a rare earth metal.
 8. A process as claimed in claim 1 wherein said charge is contained in a crucible of Y2O3 and said magnetic material is Co5R.
 9. A process as claimed in claim 1 where the strength of said magnetic field is at least 50 kilograms. 